1. Field Of The Invention
The present invention relates generally to network transmission media of the type used in industrial control, monitoring, and similar power and data network systems. More particularly, the invention relates to a novel cable structure and to a modular connector for use with such a cable. The cable and modular connector are designed for use in an industrial-type control and monitoring system in which a number of device nodes are both powered via the cable and connectors, and receive and transmit data over conductors embedded in the cable.
2. Description Of The Related Art
Various types of physical media have been proposed and are currently in use for networked control systems. Such control systems typically include a number of device nodes coupled to a set of common conductors for transmitting power and data. The node devices often include both sensors and actuators of various types, as well as microprocessor-based controllers or other command circuitry. Moreover, certain of the sensor and actuator nodes may also include signal processing capabilities, memory devices, and so forth. Power supplies coupled to the network furnish electrical energy via the network media to operate the sensors, actuators and other devices requiring an external power source. In operation, networked sensors provide information via the physical communications media relating to the states of various operating parameters. Other devices on the network process the transmitted parameter data and command operation of networked actuators, such as relays, valves, electric motors, and so forth. One device network of this type is commercially available from the Allen-Bradley Company of Milwaukee, Wis. under the commercial designation DeviceNet.
Unlike unpowered data networks, powered industrial control networks pose unique problems for the transmission of both electrical energy and data to and from networked devices. For example, the provision of power conductors and digital signal conductors in a single cable can result in unwanted noise or other interference between the conductors, ultimately leading to bit errors in the transmission of the digitized data. Such interference can result from current draws by networked devices which, depending upon the design of the network cable, can cause differential mode noise between signal conductors. Differential mode noise adversely influencing digitized information may also result from external fields, typically generated by operation of certain machines and equipment in the vicinity of the network cable and connectors. In general, such differential mode noise must be minimized to reduce the risk of the noise corrupting data transmitted to networked devices. With the increases in data transmission rates, network length and the number of devices coupled to the network, the likelihood of adverse influences of power signal changes on data signals is increased. Consequently, such internal and external noise ultimately limits the reliability of the network and networked devices, as well as limits the number of devices which can be coupled to the network and the overall length of the networked system.
Several approaches have been proposed and are currently in use for limiting the adverse influences of internal and external noise in industrial control network media. In one approach for non-powered systems, digital signal conductors are twisted in a pair to ensure that noise influencing the data signals will have similar influences on signals in both conductors, that is, that any noise will tend to be common mode noise rather than differential mode noise. Similarly, certain powered networks presently employ shielded cables in which both power and signal conductors are twisted together within a flexible metallic shields, at least partially limiting the influences of external noise and equalizing the impact of internal noise on the digitized data signals.
While network media of this type provide excellent and reliable power and data transmission capabilities, they are not without drawbacks. For example, installation of shielded network cables may be relatively time consuming, generally requiring that the shield be cut and that wires within the shield be identified, prepared and secured at each node. Where the cable is employed as a trunk line extending between a series of nodes or taps in the network, a similar procedure must be employed at each node or tap. Where the cable is continued from a node, an additional cable end must be prepared at the node.
In another powered network media system currently in use, a pair of power conductors are arranged in a cable and digitized data signals are modulated on power carried by the conductors. Networked nodes are coupled to the cable by insulation-piercing pins that make contact with the cable conductors upon installation. While this approach facilitates installation of the network, special circuitry is needed at each node point and at each power supply connected to the network to separate the digitized data signals from the power signals carried across the conductors.
Other control media are known, particularly in vehicular control system applications, wherein several conductors extend along a flat cable between networked node points. Insulation displacement pins pierce the cable jacket to make contact with the conductors at each node point. However, media of this type are generally not suitable for the communication rates and distances required in industrial network applications. Moreover, the layout of the power and signal conductors in the cable does not lead to a reduction in differential mode noise, particularly noise resulting from external sources, and may even exacerbate such noise.
There is a need, therefore, for an improved network media cable and connector system for use in industrial control networks and the like. More particularly, there is a need for a cable that includes separate power and signal conductors so as to reduce or eliminate the need for specialized circuitry at each node point for separating superimposed data signals from power signals. The cable and associated connectors should ideally provide data transmission capabilities similar to those of multi-conductor shielded cable, but facilitate installation via insulation displacement technology.